A conventional steering angle estimating apparatus for vehicle is disclosed in, for example, Japanese Patent No. 2614348 B2 (document 1). The estimating apparatus has a steering angle detecting sensor that detects a steering angle of a steering wheel, a left wheel velocity sensor that detects a velocity of the left wheels, a right wheel velocity sensor that detects a velocity of the right wheels, an estimated steering angle calculating means that calculates an estimated steering angle based on the velocity of the left wheels obtained by the left wheel velocity sensor and the velocity of the right wheels obtained by the right wheel velocity sensor, and a neutral position calculating means that calculates a neutral position of the steering angle of the steering wheel after a phase difference between a signal from the steering angle detecting sensor and the estimated steering angle obtained by the estimated steering angle calculating mean is taken into consideration.
Further, Japanese Patent Application Laid-Open No. 2003-14450 A (document 2) discloses a steering angle estimating apparatus for vehicle, which estimates a steering angle of a vehicle using a pair of rotating sensors which detect rotating states of the right and left wheels, having: a velocity detecting means that detects rotational velocities of the right and left wheels based on outputs from the paired rotating sensors; a velocity ratio calculating means that calculates a rotational velocity ratio of the rotational velocity of the left wheels to the rotational velocity of the right wheels detected by the velocity detecting means; a steering angle calculating means that calculates the steering angle of the vehicle based on the rotational velocity ratio calculated by the velocity ratio calculating means.
In document 3 (Journal of the Institute of Electrical Engineers of Japan, vol. 120, No. 2 “New tire slipping detecting method which does not require vehicle velocity information in electronic vehicle”, 2000), a motion equation of wheels and a motion equation of a vehicle body are obtained so that a slip ratio is estimated by using wheel velocities and a vehicle velocity as state variables.
However, with the technique in the document 1, only a neutral point of steering is estimated, and thus an estimation possible range is narrow. Since only a velocity of one set of the right wheel and left wheel is used for the estimation, accurate estimation cannot be made.
Since the estimating apparatus in the document 2 determines a steady state by comparing a rotational velocity ratio of two front wheels with a rotational velocity ratio of two rear wheels, a difference in turning radius between the front wheels and the rear wheels is not accurately reflected, and thus the above steady determination accuracy is not occasionally exact according to the steering angle. When only one wheel slips, the steering angle cannot be estimated.
Further, in the technique in the document 3, a torque for driving the tires should be detected, and motor driving is necessary or a driving torque sensor is necessary.
The present invention is devised in view of the above problems, and it is an object of the present invention to provide a steering angle estimating apparatus for vehicle that can accurately determine the steady state at all steering angles, and can estimate the steering angle as much as possible even in the case of a non-steady state such that one wheel slips. Its another object is to provide an electric power steering apparatus which contains the steering angle estimating apparatus for vehicle which can accurately determine the steady state at all the steering angles and can estimate the steering angle as much as possible even in a case where the steady state is not obtained.